From: Sean Christopherson seanjc@google.com
[ Upstream commit 17122c06b86c9f77f45b86b8e62c3ed440847a59 ]
Treat any exception during instruction decode for EMULTYPE_SKIP as a "full" emulation failure, i.e. signal failure instead of queuing the exception. When decoding purely to skip an instruction, KVM and/or the CPU has already done some amount of emulation that cannot be unwound, e.g. on an EPT misconfig VM-Exit KVM has already processeed the emulated MMIO. KVM already does this if a #UD is encountered, but not for other exceptions, e.g. if a #PF is encountered during fetch.
In SVM's soft-injection use case, queueing the exception is particularly problematic as queueing exceptions while injecting events can put KVM into an infinite loop due to bailing from VM-Enter to service the newly pending exception. E.g. multiple warnings to detect such behavior fire:
------------[ cut here ]------------ WARNING: CPU: 3 PID: 1017 at arch/x86/kvm/x86.c:9873 kvm_arch_vcpu_ioctl_run+0x1de5/0x20a0 [kvm] Modules linked in: kvm_amd ccp kvm irqbypass CPU: 3 PID: 1017 Comm: svm_nested_soft Not tainted 6.0.0-rc1+ #220 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 RIP: 0010:kvm_arch_vcpu_ioctl_run+0x1de5/0x20a0 [kvm] Call Trace: kvm_vcpu_ioctl+0x223/0x6d0 [kvm] __x64_sys_ioctl+0x85/0xc0 do_syscall_64+0x2b/0x50 entry_SYSCALL_64_after_hwframe+0x46/0xb0 ---[ end trace 0000000000000000 ]--- ------------[ cut here ]------------ WARNING: CPU: 3 PID: 1017 at arch/x86/kvm/x86.c:9987 kvm_arch_vcpu_ioctl_run+0x12a3/0x20a0 [kvm] Modules linked in: kvm_amd ccp kvm irqbypass CPU: 3 PID: 1017 Comm: svm_nested_soft Tainted: G W 6.0.0-rc1+ #220 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 RIP: 0010:kvm_arch_vcpu_ioctl_run+0x12a3/0x20a0 [kvm] Call Trace: kvm_vcpu_ioctl+0x223/0x6d0 [kvm] __x64_sys_ioctl+0x85/0xc0 do_syscall_64+0x2b/0x50 entry_SYSCALL_64_after_hwframe+0x46/0xb0 ---[ end trace 0000000000000000 ]---
Fixes: 6ea6e84309ca ("KVM: x86: inject exceptions produced by x86_decode_insn") Signed-off-by: Sean Christopherson seanjc@google.com Link: https://lore.kernel.org/r/20220930233632.1725475-1-seanjc@google.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/kvm/x86.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 05ca303d7fd98..68827b8dc37a5 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -8860,7 +8860,9 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, write_fault_to_spt, emulation_type)) return 1; - if (ctxt->have_exception) { + + if (ctxt->have_exception && + !(emulation_type & EMULTYPE_SKIP)) { /* * #UD should result in just EMULATION_FAILED, and trap-like * exception should not be encountered during decode.